The invention relates to means for generating a high quality picture under very low ambient light conditions, and particularly to apparatus and method for producing high quality monochrome or color television pictures under conditions of very low ambient illumination.
In the field of television, and particularly in the fields of electronic news gathering (ENG), electronic field production (EFP), surveillance, etc., there are many instances such as, for example, under twilight or heavy overcast conditions, wherein television cameras cannot be utilized successfully due to the lack of sufficient ambient light to allow recording a scene. Under such low light level conditions, news gathering or other activities using television cameras are precluded, thereby limiting outdoor use of television cameras to daylight hours, or at best, early evening situations. Likewise, closed circuit surveillance systems are limited generally to indoor or outdoor applications where there is sufficient artificial lighting to allow the use of conventional industrial television cameras.
The only known low light level cameras are various types of highly specialized surveillance devices as those used, for example, by the military. Such devices are generally monochrome systems using special image intensifier tubes, are very expensive, and under normal illumination conditions are generally incapable of making color television pictures of a quality suitable for broadcast purposes. Thus such specialized devices can be used only for their intended purposes.
The invention overcomes the shortcomings of the above mentioned devices, by providing a relatively simple system for producing high quality moving or still color television pictures, under conditions of very low ambient illumination. Further, the system is applicable to present high quality cameras utilizing conventional image pickup tubes, which cameras inherently are capable of, and intended for, generating broadcast quality pictures under normal light level conditions. Thus a particularly valuable and heretofore unavailable application of the invention combination is in the field of electronic news gathering. An ENG camera with the capability of readily being switched to a low light level mode of operation permits the capture of newsworthy events which occur outdoors in deep twilight. At present, this has been well outside the capability of all existing ENG, or of all EFP cameras. As further described below, the primary drawback of the system is that the video signal is "interrupted"; i.e., the resulting pictures which are generated do not occur at the usual frame rate, and thus are discernable by a viewer as successive pictures of the scene. However, in accordance with the invention combination, various schemes are contemplated for generating continuous video pictures from the interrupted signal. In any event, when newsworthy events occur, successive periodic pictures of the scenes are far preferable to not being able to obtain any pictures at all.
Accordingly, the invention combination utilizes the principle that the signal current generated by a television image tube, or other similar sensor, throughout a specific time interval corresponding to a specific area of the imaged scene, is directly proportional to the corresponding illumination level on the sensor and to the integration time, i.e., to the period between successive scans of the specific area. In conventional television cameras, the integration time is fixed and is equal to the reciprocal of the picture repetition frequency, i.e., to the frame frequency (1/30 of a second in the NTSC color television standard, and 1/25th of a second in the PAL standard). In the invention combination, the scanning process used in a television camera is modified to inhibit scanning for N successive frames and to enable scanning during the next frame period, in a repeating cycle of (N+1) frames duration. It follows that the signal output from the sensor during the enabled, or "on", frame will be (N+1) times larger than the corresponding signal generated by conventional scanning action. However, the signal output is zero for the N inhibited, or "off" , frames, thereby providing the interrupted video signal.
The interrupted video signal may be selectively manipulated to optimumize the final output video signal, depending upon the medium in which the video is to be used, the type or content of the video signal desired, etc. By way of example, the video signal from the camera may be recorded on a suitable video recorder and subsequently replayed in the "still frame" mode. Alternately, the video signal may be fed to a frame store, wherein the gaps in the interrupted video signal corresponding to the inhibited frames can be filled in with video generated during an adjacent enabled frame.
Accordingly, it is an object of the present invention to provide a system and method for producing high quality monochrome or color pictures under very low ambient light conditions.
It is another object to provide quality television signals under low ambient light conditions utilizing generally conventional cameras with conventional image pickup tubes.
It is still another object to provide quality television pictures in a television camera by selectively inhibiting and enabling the readout of given frames of information in a repeating cycle of frames generated by the camera.
Another object is to inhibit the readout of given frames in a repeating cycle of frames by selected manipulation of given elements of the scanning devices.
A further object is to provide optimum still or moving television pictures in low light conditions while still providing broadcast quality television signals during normal light level conditions.
A still further object is to optimize the effective sensitivity of an optical-to-electrical transducer under low ambient light level conditions.